The lithium-ion rechargeable battery technology has received a lot of attention in recent years and is used in most electronic devices today. Lithium is not a cheap or abundant metal and it is considered to be too expensive to some markets requiring rechargeable battery technologies; such as large scale stationary energy storage applications. Sodium-ion batteries are similar to lithium ion batteries, both are rechargeable and comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing energy, and they both charge and discharge via a similar reaction mechanism. When a sodium-ion (or lithium-ion battery) is charging, Na+ (or Li+) ions de-intercalate from the cathode and insert into the anode. Concurrently charge balancing electrons pass from the cathode through an external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction.
Sodium ion battery technology is considered to offer certain advantages over lithium. Sodium is more abundant than lithium and some researchers think that this will offer a solution for a low cost and durable energy storage requirement, particularly for large scale applications such as grid level energy storage.
Nevertheless a significant amount of development is required before sodium-ion batteries are a commercial reality.
There are a number of material types which have been shown to be useful in rechargeable sodium ion batteries which include; Metallate materials, Layered oxide materials, polyanionic compounds, phosphates and silicates. However, one of the most attractive classes of material is that of the layered oxides.
A well-known layered oxide material has the formula NaNi0.5Mn0.5O2. In this material the transition metal nickel is present as Ni2+ while the manganese is present as Mn3+. This is an ordered material with the Na and Ni atoms residing in discrete sites within the structure. In this case the nickel ions (Ni2+) are a redox active element which contributes to the reversible specific capacity and the manganese ions (Mn4+) play the role of a structure stabilizer. Similarly, the compound NaNi0.5Ti0.5O2 is analogous to NaNi0.5Mn0.5O2 in that the Ni2+ ions provide the active redox centre and the Ti4+ ions are present for structure stabilization. There is a considerable quantity of literature describing NaNi0.5Mn0.5O2 and the titanium analogue, as a precursor for the lithium layered oxide material LiNi0.5Mn0.5O2 and NaNi0.5Ti0.5O2 and the subsequent ion exchanging the sodium for lithium. However, recent electrochemical studies reported by Komaba et al Adv. Funct. Mater. 2011, 21, 3859 describe the sodium insertion performance of hard-carbon and layered NaNi0.5Mn0.5O2 electrodes in propylene carbonate electrolyte solutions. The results obtained show that NaNi0.5-Mn0.5O2 exhibits some reversible charging and discharging ability, unfortunately however the capacity of the material fades by 25% or more, after only 40 cycles which makes the use of this material extremely disadvantageous for rechargeable energy storage applications. As such there is significant interest in improving the electrochemical performance of such materials.
This invention discloses an oxygen non-stoichiometric material compositions based on a layered oxide framework. Herein, we define oxygen non-stoichiometry as deviation from the ABO2 formula of the layered oxide framework, wherein in a pristine material the ratio of elements is 1:1:2. Within this invention we claim that the ratio deviates from the ideal ABO2 stoichiometry in the following manner ABO2−δ wherein, the average oxidation state of one or more elements contained within the B site reduces to rebalance the structures charge and retain charge neutrality whilst the proportion of elements on the B site remains unchanged. This yields the same atomic ratios within the A and B site only with variation on the O site. So the relative proportions of elements within the material can be expressed as 1:1:2.δ and the average oxidation state of each site can be expressed as +1:+3−2/δ:−4−δ
Most of the background literature for these types of materials is based upon stoichiometric sodium transition metal oxides which adopt either a P2 or O3 layered structure. This invention describes novel compositions based on the sodium layered oxides for application in a sodium ion battery. Within this material class there is substantial prior art based on material composition, focused on the Na content and ratios of transition metal elements.
For example, US20070218361 A1 describes a material suitable for a sodium ion battery and claim a positive electrode based on a sodium-containing transition metal oxide. The compositions of the oxide is described as NaaLibMnxMy02±c, where, M may include at least one selected from the group consisting of iron, cobalt, and nickel, a is in the range 0.6 to 1.1 and b may range from 0 to 0.5, the sum of x and y may range from 0.9 to 1.1, and c may be from 0 to 0.1. In this application it is claimed that the number of oxygen atoms in the materials formula unit is based on charge compensation of the structure. In this compositional claim the O content is linked to the Na/Li content of the material and cannot be deficient i.e. there cannot be fewer oxygen atoms in the material than the sum of alkali and transition metals.
Similarly, in U.S. Pat. No. 8,835,041 B2 a layered oxide material suitable for application in an energy storage device is defined. The compositional claims of this patent are described by the formula NacLidNieMnfMz0b, wherein M comprises one or more metal cations, and the ratios of constituents are limited to 0.24<c/b=<0.5, 0<d/b=<0.23, 0=<e/b=<0.45, 0=<f/b=<0.45, 0=<z/b=<0.45, the combined average oxidation state of the metal components is in the range of about 3.9 to 5.2, and b is equal to (c+d+Ve+Xf+Yz)/2, wherein V is the average oxidation state of the Ni, X is the average oxidation state of the Mn, and Y is the average oxidation state of the M in the material. M may be selected from Mg2+, Co2+, Co3+, B3+, Fe2+, Fe3+, Ga3+, A13+, and Ti4+.
Further to these disclosures, JP2012252962A claims a positive electrode for a secondary battery, expressed by the formula: LiaAbMcOd, where A represents one or more elements selected from the group consisting of Na and K, M represents one or more transition metal elements, 0<a≦1.5, 0≦b<1.5, 0<c≦3, 0<d≦6, and 0<a+b≦1.5. The patent refers to these materials adopting a spinel type structure rather than a layered oxide structure.
Further to these documents,                U.S. Pat. No. 8,709,655B2, JP5085032B2, US20090159838A1, WO2006/057307, JP2006179473A, US20110003192A1, and WO2009/099061 detail the application of sodium layered oxides for application in energy storage devices The compositional claims of these documents limit the content of oxygen in the material to O2 and they do not contain any oxygen non stoichiometry.        